1. Field of the Invention:
This invention relates to a novel electroluminescent device.
2. Description of the Prior Art:
Electroluminescent devices utilize a substance which emits light under the influence of an electric field. Such a substance can be a powdered electroluminescent phosphor which is dispersed into a dielectric material having high dielectric constant or laminated in the form of thick layer between suitable electrodes. In an alternative form of the electroluminescent device, the electroluminescent phosphor comprises a thin film layer which is formed by vacuum evaporation. These electroluminescent devices are energized either by an alternating or direct current power source.
FIG. 1 schematically illustrates a conventional electroluminescent device dispersing a powdered electroluminescent phosphor into the cell which is driven by a dc voltage. The electroluminescent device comprises a transparent glass substrate 1 having a transparent conductive coating 2 of In.sub.2 O.sub.3 or SnO.sub.2 thereon, a sputtered electroluminescent layer 3 thereon, and an evaporated metal electrode 4 upon the electroluminescent layer 3. The electroluminescent layer 3 used in the cell is processed so as to permit some degree of dc current to pass therethrough. For instance, when using a ZnS:Mn phosphor, it is subjected to have the phosphor particles 5 coated with CuxS (1.ltoreq.x.ltoreq.2) layers 6 by immersing the phosphor particles 5 in copper sulphate or copper acetate solution as shown in FIG. 2. In the electroluminescent device shown in FIG. 1, the transparent conductive coating 2 which acts as a positive electrode and the metal electrode 4 which acts as a negative electrode are connected to a dc source which provides the power for operatng the electroluminescent device. When the power is turned on, Cu.sup.+ ions move from the Cuxs layers 6 to the inside of the phosphor particles 5 under the influence of an electric field as shown in FIG. 2, and an area without having the CuxS layers 6 is formed between the phosphor particles 5 and the transparent conductive coating 2. When a high electric field is applied to this area which is high in electric resistance, the electroluminescent layers 3 can emit light.
However, in the electroluminescent device shown in FIG. 1, the Cu.sup.+ ions in the Cuxs layers continue to diffuse in the phosphor particles 5 in the electroluminescent layer 3 contacting to the transparent conductive coating 2 under the influence of the electric field between the transparent conductive coating 2 and the metal electrode 4. As a result, the ZnS:Mn phosphor having characteristics of a n-type semiconductor undergoes doping compensation to be injected by the monovalent Cu and is converted into a phosphor having characteristics similar to those of an intrinsic semiconductor. Accordingly, when the dc voltage is applied to the resultant electroluminescent layer 3, a dim and unstable electroluminescence is observed, because the layer is high in electrical resistance and relatively small currents could pass through the layer.
In an electroluminescent device utilizing a thin electroluminescent layer, the luminescent layer is extremely thin which is about 10 .mu.m thick and is high in electric resistance. Thus, it cannot emit light of sufficient brightness unless a higher voltage is applied to the luminescent layer. Furthermore, the operating life of the device is relatively short, because pinholes are liable to be formed in the luminescent layer which cause dielectric breakdown.
In order to eliminate these disadvantages explained hereinabove, an electroluminescent device utilizing laminations of a thin film luminescent layer and a dielectric layer which is operated by an ac source has been proposed and put to practical use. However, this electroluminescent device is difficult to control, because it requires a high voltage of approximately 200 V to 400 V and complicated driving circuits for operating the device. Because of these disadvantages, the application of the electroluminescent device is limited in a certain field, and it has not been widely used as a display device, although it is highly expected its utility as a planar light emitter or a medium or large-sized planar display device.